In the manufacture of paper from wood, the wood is first reduced to an intermediate stage in which the wood fibers are separated from their natural environment and transformed into a viscous liquid suspension known as a pulp. There are several classes of techniques which are known, and in general commercial use, for the production of pulp from various types of wood. The simplest in concept of these techniques is the so-called refiner mechanical pulping (RMP) method, in which the input wood is simply ground or abraded in water through a mechanical milling operation until the fibers are of a defined desired state of freeness from each other. Other pulping methodologies include: thermomechanical pulping (TMP); chemical treatment with thermomechanical pulping (CTMP); chemi-mechanical pulping (CMP); the so-called kraft or sulfate process for pulping wood; biomechanical pulping (BMP); and sulfite-modified thermomechanical pulping (SMTMP).
These methods are also described in the patent literature. For example, Goheen et al., U.S. Pat. No. 4,145,246 teach that sulfite-modified thermomechanical pulps (SMTMP) can be formed by subjecting the lignocellulose to multistage mechanical attrition, the first stage being conducted at elevated temperature and pressure and the second stage being run under atmospheric conditions wherein a sulfite chemical is added to the lignocellulose prior to the second stage to sulfonate the lignocellulose so that a percent bound sulfur level of at least about 0.15% is provided.
Blanchette et al., U.S. Pat. No. 5,055,159 teach that biomechanical pulps (BMP) can be formed by using biological processes as a pretreatment step. The patentees disclose using a species of fungus, ceriporiopsis subvermispora, to partially digest the wood followed by mechanical pulping.
Thermomechanical pulping is also well known in the prior art. For example, Asplund, U.S. Pat. No. 2,008,898 disclose presteaming wood chips to a suitable temperature above 100.degree. C. and a corresponding pressure and refining at these conditions. Thermomechanical pulping has further been described in the literature as a process wherein the initial refining step takes place at a temperature above 140.degree. C., the lignin portion of the undelignified lignocellulose is softened so that the wood structure is broken in the lignin-rich middle lamella layer and the cellulose fibers are easily separated from each other in a substantially undamaged condition at a relatively low consumption of energy. However, subsequent fibrillation of the pulp to make the pulp useful for printing paper grades requires large amounts of energy, since, when the fibers are released intact, they are coated with the softened lignin, which on cooling reverts to a glassy state and is only, with difficulty, subsequently fibrillated. Further, the refining also causes substantial fiber length reduction thereby providing poor strength properties of the resulting product.
Imparting large amounts of power, i.e. about 55 horsepower days per ton (hpd/t), during high temperature refining has been suggested in order to make the lignin coated fibers more easily refinable. However, although such a high energy process improves fiber separation, it also causes destruction or damage to or delamination of the fiber layers themselves, thereby exposing the cellulose-rich inner surfaces to further mechanical treatment. This results in a significant reduction in the average fiber length and a concurrent reduction of strength properties.
Accordingly, none of the prior art processes teach a pulping method which produces a high yield pulp with low energy requirements. To this end, it has been found that a high yield thermomechanical pulp with low energy requirements can be obtained by the process of the present invention, thereby satisfying a long felt need in the art.